The assassination of Pierre Gemayel, a Lebanese cabinet minister and scion of a ruling Christian Maronite family, in Beirut on Tuesday has sent shockwaves through the country's establishment and is shaping the political feud raging throughout the country.

Given the timing, location and method of the killing - a sophisticated shooting in the heart of Christian east Beirut at the height of a political crisis - there is already rampant speculation as to the identity and sponsors of the assassins. That will doubtless remain the case even after the criminal investigation is complete. The consequences of this for Lebanon and the wider Middle East are already starting to become clear.

In the past 18 months, Lebanon has undergone a series of political convulsions. A major split in the political establishment and the population followed the assassination of the former prime minister Rafik Hariri in February 2005. On the one hand, a coalition of political parties coalesced in what is called the March 14 alliance (in reference to the mass protest against Syria's role in the country on March 14 2005); this is dominated by Druze and Sunni Muslim leaders (Jumblatt, Geagea and the Hariri family), as well as sections of the Christian Maronite community.

On the other hand, the opposition has grouped round the Hizbullah leader, Hassan Nasrallah, and the former Maronite general Michel Aoun, who represents large sections of the Shia Muslims and Christian Maronites and a smaller Sunni Muslim section. This fissure reflects important regional links, with the March 14 group closer to the US-Saudi Arabian sphere of influence, and the other closer, with varying degrees, to the Syrian and Iranian spheres.

This summer's US-backed Israeli war on Lebanon and Hizbullah's victory weakened the March 14 alliance and put its main players on the defensive. The opposition groups grew in strength and demanded a greater share of political power through a unity government in which they would hold a full third of the seats, and thus have the power of veto over major government decisions.

The March 14 alliance refused - and opposition groups stated their intention to use all legal and democratic means to achieve either a unity government or early national elections. Opposition figures resigned from positions of power and planned a civil disobedience campaign. The opposition was widely seen to be gaining ground, while support for the ruling March 14 alliance faltered amid confusion about its direction, and perceived ideological bankruptcy. The question was not whether the government would collapse, but when.

Meanwhile, the March 14 alliance's main foreign backer was rethinking its regional strategy. The US administration's potential shift of direction following the Republican congressional election defeat is based on dialogue with Syria and Iran. The thawing of relations between the US-EU and Syria, as well as Syria's resumption of diplomatic ties with Iraq two days ago, are steps in that direction. Rumours of a deal between Syria and the US were starting to spread.

The assassination of Pierre Gemayel could not therefore have come at a more opportune moment for the March 14 alliance. Just two days before the planned start of mass public protests, the assassination halted the opposition's momentum.

Figures associated with the March 14 alliance have filled the airwaves, reviving the slogans that originally united them 18 months ago, and reminding the public and themselves why they came into being and why they are still needed - the threat of political assassination, the drive for national independence, and protection from Syria's security apparatus and proxies.

Earlier fears about their inability to gather crowds to compete with the opposition's numbers have vanished. Blood and grief are powerful mobilising agents. Mass demonstrations scheduled for today's funeral will be their best chance of a demonstration of strength.

Furthermore, the assassination of a member of a prominent Maronite family, coupled with media attacks on Syria and opposition groups, is expected to weaken the Aoun-Hizbullah alliance, potentially swaying Christian Maronites from the opposition. In the post-assassination climate, the March 14 alliance has already extended the life and boosted the confidence of the government.

The opposition forces realise that their plans are now on hold, and they could end up accepting a negotiated solution that would have been unthinkable a few days ago. Alternatively, they may decide to sit back and ride the backwash of the assassination, waiting to regain the initiative when emotions have cooled down.

No matter who was behind the assassination of Gemayel, it is the Lebanese opposition and Syria's allies that have been hardest hit politically by this gruesome act.

· Professor Charles Harb teaches social psychology at the American University of Beirut